When we descend on a breath-hold dive, every airspace in the body is compressed by increasing pressure. The deeper we go, the smaller those airspaces become. To keep the ears and sinuses equalised, we must continuously move air from the lungs into the throat and mouth so it can be used for pressure compensation. In the shallow part of the dive, most divers do this naturally without realising it. Air flows forward into the mouth almost automatically — a process that is actually reverse...